Community Level Impact and Potential Management Practices of West Indian Marsh Grass in the Myakka River Watershed
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Community Level Impact and Potential Management Practices of West Indian marsh grass in the Myakka River Watershed Funded by The Charlotte Harbor National Estuary Program Final Project Report November 2006 Prepared by: William A. Overholt/1, Rodrigo Diaz/1, Jim P. Cuda/2, Paula Benshoff/3 /1Biological Control Research and Containment Laboratory Indian River Research and Education Center University of Florida Fort Pierce, FL 34945 /2Entomology and Nematology Department University of Florida Gainesville, FL 32611 /3Myakka River State Park Sarasota, FL 34241 Table of contents Summary ................................................................................................................................3 General Introduction ..............................................................................................................3 Impacts of Hymenachne amplexicaulis on plant and arthropod communities.......................5 Introduction........................................................................................................................5 Material and Methods ........................................................................................................5 Study area.......................................................................................................................5 Sampling ........................................................................................................................5 Statistical Analysis.........................................................................................................7 Results................................................................................................................................7 Marsh characteristics......................................................................................................7 Plant community and biomass accumulation.................................................................8 Macroinvertebrates Summer ..........................................................................................9 Macroinvertebrates Fall ...............................................................................................13 Discussion ........................................................................................................................16 Effectiveness of herbicides for controlling Hymenachne amplexicaulis in marshes...........18 Introduction......................................................................................................................18 Material and Methods ......................................................................................................18 Results and Discussion.....................................................................................................19 References cited ...............................................................................................................24 Acknowledgements..............................................................................................................25 2 Summary Invasion of exotic plants constitute a major threat to aquatic ecosystems. West Indian marsh grass, Hymenachne amplexicaulis (Rudge) Nees, is currently invading the watersheds of central and south Florida. During this process, native flora and fauna are being displaced by this aggressive grass. We investigated the impact of Hymenachne invasion to flora and macroinvertebrates in Myakka River State Park and evaluated the effect of herbicides for its control. Sites invaded by Hymenachne had larger accumulation of biomass, reduced macroinvertebrate abundance and simpler trophic structure than native sites. There was no indication that native insect herbivores were colonizing Hymenachne and this could be explained by the relatively recent arrival of this grass into Florida. The herbicide experiment suggested that Hymenachne can be controlled using a tankmix of glyphosate and imazapir, and the timing of its application should be targeted for early growing season (February to May) when the plant is under stress and before flowering. Management of this invasive grass in infested wetlands must include a combination of strategies such as winter burning, herbicide application and hydroperiod control. Prevention by monitoring and early control is ideal in uninfested wetlands. General Introduction Invasion of exotic species poses a serious threat to Florida’s sensitive ecosystems. International trade, tourism, agricultural and urban disturbance have increased the probability of establishment of exotic plants. In Florida wetlands, exotic species spread rapidly due to floods, large interconnected waterway systems, and increased use of commercial and recreational boats. Fertilizer and sediment runoff from agricultural lands and waste water from beef and dairy operations contribute to successful establishment of aquatic exotic plants. Wunderlin and Hansen (2003) reported 1,316 exotic plants species naturalized in Florida, with 125 species being serious threats to natural areas (FLEPPC 2005). Of those, 65 are considered highly invasive because they are disruptive to native plant communities. West Indian Marsh Grass, Hymenachne amplexicaulis (hereafter referred as Hymenachne), is one of many species currently invading sensitive wetlands in Florida. Hymenachne is a native of South America and the West Indies and has spread to most countries of the neo-tropics. The pathway and timing of the introduction of this grass into Florida are uncertain; however, the first herbarium record was from a ponded pasture in Palm Beach County in 1957 (University of Florida Herbarium). This suggests that the grass could have been intentionally introduced as forage. The next record was from a wet pasture in Collier county in 1977 (University of Florida Herbarium). Current records confirm that Hymenachne is present in wetlands and rivers in 16 counties in Florida. Hymenachne is present in most the counties included in the Charlotte Harbor National Estuary Study Area (University of Florida Herbarium and personal communication with aquatic plant managers) (Fig. 1). Large monocultures of 3 Hymenachne can be found in the rivers, canals and wetlands located in the Myakka and Peace River Basins. Nutrient enrichment, especially with nitrogen and phosphorous, of surface water due to runoffs from agricultural fields and geological deposits of phosphate may have facilitated the establishment and dominance of Hymenachne in these rivers (Charlotte Harbor Environmental Center, 2002). Counties infested with Hymenachne amplexicaulis Figure 1. Current distribution of Hymenachne amplexicaulis in Florida. Invasion of Hymenachne is favored by aggressive mechanisms of reproduction and dispersal. In Australia, a single inflorescence can produce more than 4000 seeds (Tropical Weeds Research Centre 2003) with approximately 98% viability (Lyons 1996). Another method of reproduction is through vegetative material (stolons). Hymenachne experimental colonies in the Indian River Research & Education Center at Fort Pierce are easily reproduced by planting small pieces of stems containing at least one node. Moreover, ponded pastures in Australia were easily established by simply casting pieces of the grass from boats (Lukacs 1996). Summer floodwaters in Florida can transport Hymenachne seeds and stolons great distances through watersheds complicating management programs. Simplification of wetland ecosystems due to the invasion of Hymenachne could have severe impacts in the native fauna. Diverse aquatic habitats are places for feeding, resting, refuge and reproduction for wading birds, wood stork, snail kite, killifishes, live bearers, juvenile sunfishes, southern leopard frog, pig frog, green tree frog, American alligator and American crocodile, among others (Mitsch and Gosselink 2000). The present study had two objectives a) assess the impact of Hymenachne on the flora and macroinvertebrate fauna and b) evaluate the use of herbicides as a means to control Hymenachne. 4 Impacts of Hymenachne amplexicaulis on plant and arthropod communities Introduction Large infestations of exotic grasses can reduce biodiversity in aquatic ecosystems. Recent studies in wetlands demonstrate that exotic grasses are capable of simplifying the plant diversity and reducing or changing the arthropod community (Herrera and Dudley 2003, Houston and Duivenvoorden 2002, Talley and Levin 2001, Posey 1988). These changes can be linked to alteration of trophic structure, and habitat usage by birds, fish and other vertebrates. Despite the large areas infested and visible reduction of wetland plants, no studies have been conducted to quantify the impact of Hymenachne on Florida native plant and arthropod communities. The objectives of this study were to quantify the impact of Hymenachne on native flora and macroinvertebrates assemblages in floodplain marshes. Material and Methods Study area The Myakka River flows through 45 square miles of Myakka River State Park, which is located in Sarasota County in southwest Florida. The land cover in the upper river basin is dominated by a mosaic of pastures, hardwood forest, palms, citrus groves and row crops. Rainfall is seasonal with most of the rain falling between April and October (Kushlan 1990). Heavy rain triggers floods during the summer in the park. In the last four years, increased hurricane activity generated large discharges of water into marshes next to the river. Vegetation of marshes adjacent to the Myakka River is composed of: (a) emergent